CHAP, iv.] THE SKELETON OF THE LIMBS. 121 



which take place between certain hones of the skull where, as we 

 have seen, an interdigitation of the processes of their irregularly- 

 shaped margins takes place, forming what is called a dentated or 

 serrated suture (as between the parietals or frontals) ; or (2) such as 

 those between the upper jaw-bones, where there is no inter- 

 digitation, and where the adjoined even edges form what is termed 

 an harmonia or false suture ; or (3) such as that formed by the 

 temporal bone with the parietal, where the adjacent margins are 

 bevelled off and one overlaps the other, forming what is called a 

 squamous suture ; or (4) such as that between the ethmoid and the 

 vomer, where a ridge of one bone is received into a groove in 

 another which is called schindylesis ; or finally (5) where one hard 

 part is received into the cavity of a bone, as the teeth fit into the 

 jaws, a mode of union named gomphosis. 



In all the immovable joints no cartilage is interposed, there being 

 only periosteum, such as coats all bones. 



The great majority of the bones, however, are intended to move 

 one upon another, and are on that account joined together by means 

 of some other and thicker substance than the periosteum. 



The MIXED or imperfect joints (amphiarthroses) have, however, 

 very little mobility, and their adjacent parts do not present smooth 

 surfaces, but are connected by fibrous tissue or continuous cartilage, 

 or both. 



Examples of such joints are to be found in the junction of the 

 haunch-bones with the spine, or in that formed by the bodies of the 

 vertebra one with another. These latter are connected, as we 

 have seen, * by discs, each consisting, towards its circumference, of 

 fibres running obliquely upwards and downwards, and in its middle, 

 of a pulpy substance containing many cartilage corpuscles. These 

 pads, while allowing very little movement between any two adjacent 

 bones, give a considerable amount of mobility to the whole series. 

 They also serve to prevent shock. 



In all MOVABLE or complete joints (diarthroses) the opposed ends 

 of the bones are covered each with its own distinct, separate, and 

 very smooth cartilage, and thus the bones can glide freely on each 

 other. 



In addition to this, however, each complete joint is provided (as 

 has been already indicated in various instances) with a kind of bag 

 containing a viscid fluid. This bag, at first complete, comes in 

 adult life to be attached to each of the cartilaginous surfaces^ 

 near its border, and to disappear altogether over the central part of 

 each such cartilage. By the fluid thus interposed between the 

 cartilages all friction is avoided, and a perfectly smooth, easy, and 

 even motion is provided for. 



The fluid is termed SYNOVIA, and is an albuminous liquid commonly 

 called "joint-oil," yet it is not really of an oily nature, although it 

 contains some fatty matter. 



* See ante, p. 52. 



